ASK DOCTOR K: Long-term side effects of Pradaxa unknown

DEAR DOCTOR K: My wife has atrial fibrillation. Her medication was recently changed from warfarin to Pradaxa. Her doctor says the new medicine does not require regular INR tests and is just as effective. Is that so?

In people with atrial fibrillation, the upper chambers of the heart (the atria) lose that strong beating action that keeps the blood moving efficiently. As a result, blood tends to pool in the atria. When it does, clots are more likely to form.

If a blood clot gets pumped out of the heart and lodges in a blood vessel in the brain, it can cause a stroke. Clots that get stuck elsewhere can also cause serious problems. Anticoagulant drugs like warfarin (Coumadin) and dabigatran (Pradaxa) reduce the tendency for blood to clot.

Warfarin is an important anticoagulant medicine, but the dosing is tricky. If the dose is too low, clots form. If it's too high, bleeding problems may result. The international normalized ratio (INR) is a blood test. Anyone on warfarin must have regular INR tests to ensure their dose is correct.

To make matters worse, the body can change over time in how it reacts to the warfarin. One month the dose is having just the right effects, but the next month the dose is either not strong enough or is too strong. Because of this variability, warfarin is the drug most responsible for causing emergency hospitalizations from drug side effects in people over age 65.

Pradaxa appears to be just as effective as warfarin at preventing blood clots. It is probably more effective at preventing strokes, and probably causes fewer cases of bleeding in the brain (a side effect of any anticoagulant). What's more, its effect on the blood is more predictable than warfarin's. As a result, regular INR tests aren't necessary. That's a big advantage.

There are, however, two risks to Pradaxa. First, the drug has a short half-life: It is largely out of your system in less than 24 hours. If you miss a dose, you have an increased risk of developing a blood clot compared to if you missed a dose of warfarin. So it's really important to remember to take the pill every day.

The second risk is that treatments to rapidly reverse the effects of Pradaxa are unproven. It's easy to reverse the effects of warfarin. Why would you want to rapidly reverse the effects of either drug? If you were injured and started bleeding profusely, for example, you'd want to quickly, if temporarily, reverse the anti-clotting effects of the drug.

Another issue with Pradaxa is expense: It costs a lot more than warfarin. And because it's new, we don't have good information on any possible long-term side effects.

Anticoagulant drugs that don't require regular blood tests are quite attractive. Not surprisingly, there are several in the pipeline. Rivaroxaban (Xarelto) is approved in the United States to prevent blood clots in the legs, and in people having certain types of surgery of the legs. It has not yet been approved for atrial fibrillation.